Strontium (Sr) is not a transition metal; it is classified as an alkaline earth metal. The periodic table places Strontium into a distinct family of elements with predictable characteristics that are fundamentally different from those of transition metals.
Identifying Strontium on the Periodic Table
Strontium has atomic number 38, placing it in Period 5 and Group 2 of the periodic table. Elements in Group 2 are collectively known as the alkaline earth metals. Strontium is a soft, highly reactive metal that exhibits a characteristic silvery-white or slightly yellowish sheen when freshly cut. Because of its high reactivity, it is never found in its pure state in nature.
The chemical behavior of Strontium is dominated by its strong tendency to lose two electrons to achieve a stable electron configuration. This results in the formation of an ion with a fixed \(+2\) oxidation state, a trait common among all alkaline earth metals. This predictable, single oxidation state is one of the key indicators that Strontium belongs to the main-group elements, rather than the d-block metals.
Defining the Characteristics of Transition Metals
Transition metals occupy the central block of the periodic table, spanning Groups 3 through 12. These elements are characterized by a set of physical and chemical properties that distinguish them from the alkaline earth metals. They are generally harder, stronger, and have higher melting and boiling points. Many transition metals are also far less reactive than the alkaline earth metals, which contributes to their usefulness in structural applications.
Chemically, a defining feature of transition metals is their ability to exhibit multiple, variable oxidation states, such as iron forming both \(\text{Fe}^{2+}\) and \(\text{Fe}^{3+}\) ions. This variability allows them to form a wide array of compounds, many of which are distinctively colored. Transition metals are also frequently employed as catalysts due to their unique electron structure.
The Basis for Strontium’s Classification
The fundamental distinction between Strontium and a transition metal lies in their respective electron configurations. Strontium’s neutral atomic state has an electron configuration that ends in \(5s^2\). This means its two outermost valence electrons completely fill the \(5s\) orbital, placing it in the s-block of the periodic table. When Strontium forms its ion, it loses these two \(5s\) electrons, resulting in the \(\text{Sr}^{2+}\) ion with a stable noble gas configuration.
A true transition metal is defined by having a partially filled \(d\)-orbital in at least one of its common oxidation states. These \(d\)-orbitals are the reason for the transition metals’ characteristic properties, such as their multiple oxidation states and the formation of colored complexes. Strontium’s chemical behavior is governed by the \(s\)-orbital filling and subsequent loss of these electrons, which leaves the inner \(d\)-orbitals completely filled. Therefore, the simple, stable \(\text{Sr}^{2+}\) ion confirms its position as an alkaline earth metal and not a member of the transition metal series.